Walls that emulate nature

As thermodynamic systems, my cat and I are marvels of engineering. We produce our own heat, sense warmth and cold, can close our pores to prevent moisture loss, shed moisture to keep cool, and more. The cat fluffs up or flattens its fur to vary its thermal insulation and I vary layers of clothing to do the same. The result is comfort and efficient energy management. We can’t expect a house to similarly adapt to changing conditions but we can expect it to provide sustained comfort. When that comfort is lacking or costs too much, there is no one but ourselves to blame: houses are human creations. We used to build them to be “affordable to buy”, leaving the cost of comfortably living in them up to the buyer. When heating costs were low and no one worried about carbon emissions fouling the climate, we accepted this bargain. We can no longer. Today we want our houses to be affordable not just to buy but to live in. As discussed in a recent column, Passivhaus is the state of the art design-build system that best satisfies both requirements.

Passivhaus is a new and different building system. For the first time in the history of construction, it introduces information technology as an integral part of the design-build process, not just as a design tool, but also as a tool of modeling and optimizing any building for not just first cost but also for lasting energy efficiency and overall performance. The typical Passivhaus wood-frame wall sandwich, for instance, consists of not one but two functional layers:

(1) An inner frame wall, built of 2x4s or 2x6s, accommodates utilities, like wiring, electrical outlets and switches as ususal. The outer surface is structural-grade oriented strandboard (OSB) with all seams sealed airtight with tape. This functions as both a structural element and an air/moisture barrier, which is taped to the air/vapor barriers of the concrete slab or basement and the topmost ceiling or roof to form a continuous barrier around the heated building envelope. This inner wall is left unfinished on the inside, until the air-tightness of the complete structure can be tested with a blower door. Only after any air leaks have been fixed are insulation and sheetrock added.

(2) A superinsulated outer wall is attached to the outside of the inner frame-wall. This may be (a) rigid foam insulation applied to the OSB of the inner wall (or, in place of the OSB, structural insulated panels may be applied to the inner framing, tape-sealed together to form a continuous layer). (b) A second stud wall may be constructed outside the inner wall, creating a cavity between two walls, to be filled with insulation. (c) An outer wall constructed using engineered-wood I-joist as “studs” attached to the outside of the inner wall. The distance between the two walls or the size of the I-joists determines the thickness (and R value) of the insulation, as optimized during the modeling/ design phase. I favor (c) above because it uses the least amount of wood and fossil-derived chemicals. (On the other hand, it results in thicker, more massive walls.)

Option (a) relies on a vapor-impermeable, thick exterior rigid foam layer to prevent the airtight OSB layer’s temperature ever to drop to the dew point, preventing condensation inside the structure. In (b) and (c) the outer, superinsulated section is designed to dry toward the outdoors. Accordingly, a covering of a heavy duty building fabric is applied to the outside of the outer framing, taped together to make it continuous. This fabric sheds rain, but allows moisture to permeate from inside the wall to the outside. Next vertical 1x3" wooden battens are attached to the outside of (a), to accommodate horizontal siding. In case of (b) and (c) horizontal battens are attached first, tying together the outer edges of the framing, with vertical battens and siding to follow. The vertical battens provide room for drainage as the outer wall dries out behind the siding. In all three wall systems, the inner stud-wall dries toward the indoors. Once the building passes the air tightness test, both wall cavities are blown full of dense-packed cellulose insulation through carefully cut blow holes, which are then hermetically sealed. The interior is finished as usual. In our Maine climate, a superinsulated Passivhaus wall usually has an R-value of 55 to 60.

Attics and roofs in Passivhaus buildings are designed to accommodate substantial amounts (2' or more) of cellulose insulation. The air/vapor barrier here – tape-sealed to the OSB barrier of the walls – may also be taped-together OSB or a continuous vapor-impermeable building fabric attached to the underside of ceiling joists or roof trusses.

The above techniques may also be adapted to existing buildings. More on that in a future column.